Process and apparatus for forming shell molds

Abstract

 A pattern holder (12) and a sand tank (14) are rotatably mounted on a frame (16) so that the pattern holder can be inverted over and sealably mounted atop the sand tank. A vented pattern (26) is secured to the pattern holder and forms a first chamber (38) between one side of a pattern (26) and the interior of the pattern holder. A second chamber (90) is formed between the other side of the pattern and the interior of the sand tank when the pattern holder adjoins the sand tank. O-ring seals (32) and (80) isolate the chambers from the atmosphere so that a partial vacuum can be formed therein. A mixture (84) of sand and a gas-curable binder is provided in the sand tank so that the mixture will fall onto the pattern (26) upon inverting the sand tank over the pattern holder. A flexible line (42) is provided for controllably introducing a gas catalyst into the first chamber (38) in order to cure a portion of the sand-binder mixture to form a shell mold (94) of a predetermined thickness on a pattern face (36) of the pattern. Flexible lines (41) and (43) are provided for evacuating and purging the first chamber (38) at different stages during the process.

Description

[0001] The present invention relates to an apparatus and process for forming shell molds which are used to make metal castings, the shell molds being formed from a mixture of sand or a gas-curable binder according to the so-called "cold box" technique.

[0002] It is an aim of the present invention to provide an improvement over the basic "cold box" process described in U.S. Patent No. 4,089,363, the terms of which are incorporated by reference herein insofar as they are not inconsistent with the description which follows.

[0003] According to one aspect of the present invention, there is provided apparatus for forming shell molds by passing a gas catalyst through vents in a pattern to cure a predetermined portion of a mixture of sand and binder disposed on a shell-forming surface of the pattern, the apparatus comprising: a pattern holder adapted to retain the pattern and form a first chamber therewith; first means for controllably introducing gas into and withdrawing gas from the first chamber; a sand tank having walls defining a second chamber for containing a quantity of the sand-binder mixture, the walls terminating in a rim defining an opening at an upper end of the sand tank; second means for inverting the pattern holder over the sand tank to align the pattern with the opening; third means for sealing the pattern holder atop the sand tank to isolate the first and second chambers from the atmosphere and from each other except for gaseous communication between the first and second chambers via the vents in the pattern; and fourth means for inverting the sand tank over the pattern holder to permit sand contained within the second chamber to fall onto the shell-forming surface; whereby the first means can be actuated to introduce a gas catalyst into the first chamber to form a shell mold in the portion of the sand-binder mixture adjacent to the shell-forming surface.

[0004] According to another aspect of the present invention, there is provided a process for forming shell molds by passing a gas catalyst through vents in a pattern mounted on a pattern holder forming a first chamber therewith to cure a predetermined portion of a mixture of sand and binder disposed on a shell-forming surface of the pattern, first means being provided for controllably introducing gas into and withdrawing gas from the first chamber, the process comprising, sequentially: providing a mixture of sand and a gas-curing binder in a sand tank; sealably mounting the pattern holder atop the sand tank to form a second chamber defined between the vented pattern and the interior of the sand tank, the first and second chambers communicating through the vents in the vented pattern; forming a partial vacuum in the first and second chambers by withdrawing gas from the first chamber; inverting the joined sand tank and pattern holder to allow the mixture to fall onto the shell-forming surface of the vented pattern; vibrating the sand tank to assure proper distribution and density of the mixture over the shell-forming surface; introducing a gas catalyst into the first chamber to cure a predetermined amount of the mixture adjacent to the vented pattern to form a shell mold of a predetermined thickness; withdrawing the gas catalyst from the first and second chambers to prevent further curing; inverting the joined sand tank and pattern holder to allow the uncured mixture to fall away from the shell mold; returning the first and second chambers to atmospheric pressure; separating the pattern holder from the sand tank; and removing the shell mold from the shell-forming surface.

[0005] The advantages offered by the invention include an apparatus for the efficient and effective production of shell molds using a "cold box" process.

[0006] A further advantage of the invention is that is provides a process for forming shell molds from a mixture of sand and binder sand in a manner in which the portion of the sand and binder mixture not used in one shell-forming cycle is reclaimed for use in subsequent process cycles.

[0007] An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURES 1 - 5 are vertical cross-sections through apparatus embodying the invention and schematically depict respective stages in a process for forming shell molds in accordance with the present invention;

FIGURE 6 is a plan view of a typical vented pattern secured to a portion of the apparatus;

FIGURE 7 is a perspective view of a typical vented insert element, a plurality of which are contained in the vented pattern; and

FIGURE 8 is a perspective view of a deflector plate employed in a modified form of the preferred apparatus.

[0008] With reference to Figure 1, an apparatus for forming shell molds in accordance with the invention is illustrated and designated generally by reference numeral 10. Figure 1 shows the apparatus 10 in an open position during an initial stage of a process in accordance with the invention. The apparatus 10 includes a pattern holder 12 and a sand tank 14 which are pivotally supported at a common axis by a frame 16, shown only partially for convenience of illustration.

[0009] The pattern holder 12 comprises a case 18 rimmed by an annular flange 20 which defines an opening 22. The flange 20 has a flat upper surface 24 to which a vented pattern 26 having a peripheral flange 28 is secured in a suitable manner, for example by means of threaded fasteners 30. A conventional O-ring 32 adapted to sealingly engage the peripheral flange 28 inwardly from the threaded fasteners 30 is carried in a recess in the annular flange 20. The pattern 26 has an interior portion 34 supported over the opening 22 by the peripheral flange 28. It will be appreciated that the interior portion 34 can assume various desired shapes for producing different shell molds in accordance with the process to be described below. The upper surface of the interior portion 34 defines a shell-forming surface or pattern face 36. A gas dispersion chamber 38 is defined by the interior surfaces of the case 18 and pattern 26. A manifold 40 is secured to the bottom of the case 18 as a means for introducing flexible tubes or gas lines 41, 42 and 43 into the chamber 38.

[0010] Referring to Figure 6, it will be seen that the pattern 26 includes a plurality of vented insert elements 50 disposed in a uniform arrangement throughout the interior portion 34. The preferred insert element 50, which is illustrated separately in Figure 7, has a cylindrical body and a plurality of slotted vents 52 passing therethrough. Such vented insert elements 50 are conventional in the art. Supporting the pattern holder 12 are two pivot arms 60 and 6l which are spaced apart as seen in Figure 6 and are journaled on a pivot pin 64 carried in the frame 16.

[0011] When the pattern holder 12 is in its upright position as depicted in Figure 1, the pivot arms 60 and 61 abut a rest 66 which is supported by or forms a part of frame 16. Similarly, the sand tank 14 is supported by a pivot arm 70 journaled on the pin 64 between the arms 60 and 61, the arm 70 abutting a rest 72 on the frame 16 when the tank 14 is in its upright position as depicted in Figure 1.

[0012] The tank 14 comprises a bucket-like case 74 having an annular rim 76 defining a large diameter opening 78 at its upper end. Carried in a recess in the rim 76 is a conventional 0-ring 80 adapted to sealingly engage the peripheral flange 28 when the pattern holder 12 and sand tank 14 are brought together as shown in Figure 2. Secured to the bottom of the sand tank 14 is a conventional vibrator 82, the purpose of which will become apparent from the description which follows.

[0013] In accordance with the preferred process, a suitable gas-curable binder, such as a phenolic or an isocyanate resin in a solvent, is mixed with foundry sand in accordance with known "cold box" techniques. The tank 14 is then partially filled with the sand-binder mixture as indicated by reference numeral 84 in Figure 1. The pattern holder 12 is then rotated about the pivot pin 64 to an inverted position as depicted in Figure 2 wherein the peripheral flange 28 is seated on the annular rim 76 and against the O-ring 80, thereby forming an air-tight seal therewith and defining a second chamber 90 between the pattern 26 and the interior of the tank 14. At this stage in the process, the seals formed by the 0-rings 32 and 80 isolate the dispersion chamber 38 and the second chamber 90 from the atmosphere and from each other except for gaseous communication between chambers via the vents 52 in the pattern 26. In an alternative arrangement contemplated by the invention, a modified pattern 26 could be mounted entirely within the annular flange 20 and sealed against the surfaces that define the opening 22. In such an arrangement, the annular flange 20 could be seated directly on the annular rim 76, thus eliminating the need for one of the two O-rings 32 and 80 along with the respective recess in the flange 20 or rim 76. It will be appreciated that the invention is adaptable to other sealing arrangements suitable for isolating the chambers 38 and 90 from the atmosphere.

[0014] Next, a partial vacuum is established in the chambers 38 and 90 by pumping gas out through the line 41. In accordance with an important advantage of the invention over the prior art systems, gas flows freely from the second chamber 90 into the dispersion chamber 38 through the vents 52 without interference from the sand-binder mixture 84. The system described in U.S. Patent No. 4,089,363 necessitates forming a vacuum while the sand-binder mixture is in contact with the vented pattern, thus requiring more pumping effort and tending to clog the vents in the pattern.

[0015] Once the desired pressure difference between the chambers 38 and 90 and atmospheric pressure has been reached, the preferred pressure being about 15 in. Hg. (5.08 x 10 Pa) below atmospheric pressure, the sand tank 14 and adjoining pattern holder 12 are rotated about the pivot pin 64 to invert the sand tank 14 over the pattern holder 12 as depicted in Figure 3 so that the sand-binder mixture 84 will fall onto the pattern face 36. The vibrator 82 is then activated in order to pack and distribute the mixture 84 evenly against the pattern face 36. Performing the vibration step after evacuating the chambers 38 and 90 is believed to produce improved shell integrity.

[0016] Next, a gas catalyst is introduced through line 42 until the pressure difference between the chambers and atmosphere has been reduced to a predetermined level, preferably about 6 in Hg. (1.69 x 10 Pa). A suitable gas catalyst for use with a binder of phenolic resin or isocyanate resin is dimethylethylamine introduced via a suitable carrier gas such as carbon dioxide. A portion of the sand-binder mixture 84 adjacent to the pattern 36 is hardened by exposure to the gas catalyst. The level of gas penetration, and thus hardening, which is depicted by the dashed line 92, is determined by controlling the pressure change brought on-by the entering gas catalyst. Once the sand-binder mixture has hardened to the desired level, line 42 is closed and the gas catalyst is withdrawn from the chamber 38 through line 41. In so doing, the chamber 38 is pumped back down slightly beyond the initial pressure difference, which in this example is 15 in. Hg. (5.08 x 10 4 Pa) below atmospheric pressure. Next, the sand tank 14 and the pattern holder 12, still sealably joined together, are rotated to the position shown in Figure 4. This allows the non-hardened mixture 84 to fall to the bottom of the sand tank 14 leaving a shell mold 94 of resin-bonded sand adhering to the pattern face 36. Then, the chamber 38 is preferably purged with dry air introduced through line 43. The chamber 38 is then returned to atmospheric pressure and the pattern holder 12 is separated from the sand tank 14 and rotated back to its upright position as shown in Figure 5. The shell mold 94 is then separated from the pattern face 36 in a suitable manner, such as by means of conventional stripping pins (not shown). The apparatus 10 is then ready to begin the next process cycle.

[0017] A modification of the apparatus 10 will now be described with reference to Figure 8. Depending on the velocity of the gas catalyst entering the chamber 38 through line 42, it may be desirable to provide a deflector 96 for diverting the flow of gas catalyst from its entry direction to several lateral directions for improved dispersion. Use of the deflector 96 is advantageous at relatively high flow rates which might otherwise produce a shell mold having an uneven thickness. It will be appreciated, therefore, that the deflector 96 permits even curing at such relatively high flow rates, thereby reducing production tine.

[0018] Other advantages of the invention will be apparent to those skilled in the art. For example, the rotatable feature of the pattern holder 12 and sand tank 14 greatly improves production efficiency. Contrary to certain prior art techniques, it is not necessary to heat the pattern 26, thus conserving energy. It will be readily apparent that chemicals and other materials are also conserved by a process embodying the invention.

Claims

1. Apparatus for forming shell molds by passing a gas catalyst through vents in a pattern (26) to cure a predetermined portion of a mixture of sand and binder (84) disposed on a shell-forming surface (36) of the pattern, the apparatus comprising: a pattern holder (12) adapted to retain the pattern and form a first chamber (38) therewith; first means for controllably introducing gas into and withdrawing gas from the first chamber; a sand tank (14) having walls defining a second chamber (90) for containing a quantity of the sand-binder mixture (84), the walls terminating in a rim (76) defining an opening (78) at an upper end of the sand tank; second means for inverting the pattern holder over the sand tank to align the pattern with the opening; third means for sealing the pattern holder atop the sand tank to isolate the first and second chambers (38, 90) from the atmosphere and from each other except for gaseous communication between the first and second chambers via the vents in the pattern; and fourth means for inverting the sand tank over the pattern holder to permit sand contained within the second chamber to fall onto the shell-forming surface (36); whereby the first means can be actuated to introduce a gas catalyst into the first chamber to form a shell mold (94) in the portion of the sand-binder mixture adjacent to the shell-forming surface.

2. Apparatus according to claim I wherein the first means comprises first and second lines (41, 42) having ends secured to the pattern holder (12) to permit gaseous communication between the lines and the first chamber (38), the first line (41) serving to withdraw gas from the first chamber to permit the formation of a partial vacuum therein, the secondline (42) serving to introduce the gas catalyst into the first chamber.

3. Apparatus according to claim 2 wherein the first and second lines (41, 42) are flexible and otherwise adapted to permit the pattern holder (12) to be inverted without resistance from the lines and without disconnecting the lines from the pattern holder.

4. Apparatus according to claims 2 or 3, wherein the first means further comprises a third flexible line (43) which serves to introduce air into the first chamber (38) for purging the gas catalyst therefrom.

5. Apparatus according to any preceding claim, and further comprising means (96) disposed in the first chamber (38) for deflecting the flow of gas catalyst laterally in several directions within the first chamber.

6. Apparatus according to any preceding claims, wherein the second and fourth means comprise pivot arms (60, 61) having a common pivot axis.

7. Apparatus according to any preceding claims, wherein the third means comprises a flexible sealing member (80) carried in the rim of the sand tank (14) and adapted to sealingly engage a surface (28) of the pattern or the pattern holder.

8. Apparatus according to any preceding claim and further comprising means (82) for vibrating the sand tank to assure proper distribution and density of the sand-binder mixture (84) on the shell-forming surface (36).

9. A process for forming shell molds by passing a gas catalyst through vents in a pattern (26) mounted on a pattern holder (14) forming a first chamber (38) therewith to cure a predetermined portion of a mixture of sand and binder (84) disposed on a shell-forming surface (36) of the pattern, first means being provided for controllably introducing gas into and withdrawing gas from the first chamber, the process comprising, sequentially: providing a mixture of sand and a gas-curing binder in a sand tank; sealably mounting the pattern holder (14) atop the sand tank (12) to form a second chamber (90) defined between the vented pattern (26) and the interior of the sand tank, the first and second chambers communicating through the vents in the vented pattern; forming a partial vacuum in the first and second chambers by withdrawing gas from the first chamber (38); inverting the joined sand tank and pattern holder to allow the mixture (84) to fall onto the shell-forming surface (36) of the vented pattern; vibrating the sand tank (12) to assure proper distribution and density of the mixture over the shell-forming surface; introducing a gas catalyst into the first chamber (38) to cure a predetermined amount of the mixture adjacent to the vented pattern to form a shell mold (94) of a predetermined thickness; withdrawing the gas catalyst from the first and second chambers to prevent further curing; inverting the joined sand tank and pattern holder to allow the uncured mixture to fall away from the shell mold (94); returning the first and second chambers to atmospheric pressure; separating the pattern holder (12) from the sand tank (14); and removing the shell mold (94) from the shell-forming surface (36).

10. A process according to claim 9, and further comprising purging the first and second chambers with dry air after inverting the joined sand tank (14) and pattern holder (12) to allow the uncured mixture to fall away from the shell mold (94).

Drawing